public CreateUserViewModel() { CreateCommand = new RelayCommand(() => { if (Username == null) return; if(Username.Trim().Length<3) { Notifier.ShowMessage("Please enter at least 3 character for a user name!"); return; } if(StudentRepository.GetStudents().Select(s=>s.Username).Contains(Username)) { Notifier.ShowMessage($"{Username} already exist. Please specify a different one."); return; } Student student = new Student() { Username = Username }; StudentRepository.Add(student); StudentRepository.Logged = StudentRepository.GetStudents().Where(s => s.Username == student.Username).First(); PerfomanceMonitor.PopulateLetters(); LessonRepository.PopulateUserLessons(); Notifier.ShowMessage($"{Username} created successfully"); DialogService.HideDialog(GetType()); DialogService.ShowDialog(typeof(MainVM)); DialogService.CloseDialog(GetType()); }); CancelCommand = new RelayCommand(() => DialogService.CloseDialog(GetType())); }